In the various applications for diffusing gas into liquids, such as in the aeration of wastewater, it is known that the highest efficiency is achieved when the gas is released as fine bubbles. The efficiency in transferring oxygen or another gas to the liquid is enhanced by maximizing the bubble surface area compared to the volume. Consequently, the gas transfer efficiency increases directly with decreasing bubble diameter, so fine bubbles result in a more efficient process.
Fine bubble technology has made use of tube diffusers that include a flexible membrane sleeved onto a tube diffuser body and provided with small perforations for discharging the gas into the liquid. When gas pressure is applied inside of the membrane, the membrane is expanded and the perforations open to discharge gas through them in the form of fine bubbles. When the gas pressure is relieved, the membrane collapses on the diffuser body and creates a seal that prevents liquid from leaking into the diffuser tubes. An example of a tubular membrane diffuser of this type is found in U.S. Pat. No. 4,960,546 to Tharp.
Disk and panel diffuser units have also been used in the aeration of wastewater and other gas diffusion operations. In a disk or panel diffuser, a flexible membrane overlies a chamber in the diffuser body and expands when gas pressure is applied to the chamber. Perforations in the membrane then open to discharge fine bubbles of gas. The perforations close when the gas pressure is relieved, and the membrane collapses onto the base of the air chamber or backer plate.
The panel diffuser makes use of a flat membrane bonded or otherwise secured to a frame which provides a plenum beneath the membrane. The membrane typically has perforations arranged in rows for discharge of the gas supplied to the plenum. The panel diffuser is functionally similar to the disk diffuser and differs principally in that it has a rectangular geometry rather than a round disk shape as is the case with a disk diffuser.
Although these membrane diffusers function well for the most part, they are not wholly free of problems. When gas pressure is applied, the membranes deflect unevenly. In the disk and panel, the membrane is fixed at its outer edges, so there is a dome effect created with the center of the membrane being at a higher elevation than the rim area. The perforations near the center discharge more gas because they are submerged to a lesser extent than the rim and thus subjected to a reduced static pressure head. The uniformity of the air distribution thus suffers, and the gas transfer efficiency decreases with the decrease in the uniformity of the gas distribution over the surface of the membrane. The greater deflection of the center area of the membrane may also result in the perforations there opening to a greater extent, and this may aggravate the lack of uniform gas release. Panel diffusers are subject to the same problems as disk diffusers as to the non-uniformity of the gas distribution caused primarily by the differential in elevation between the center area and the edge areas when the membrane is deflected less than the center area.
Due to the ability of a tubular shape to resist stress, there is little deflection in a tubular membrane. Nevertheless, the top of the tube is at a higher elevation and subject to less pressure head, so it discharges more gas than the bottom or sides. Again, this detracts from the uniformity of the distribution over the membrane surface and results in a lower gas transfer efficiency than in the case of more uniform distribution.
This non-uniformity has been partially addressed in disk diffusers by tapering the membrane such that its thickness decreases toward the outer edges. The resistance to gas flow through the perforations is thereby decreased near the edges and counteracts to some extent the effect of the greater deflection at the center. However, non-uniformities are still present and this technique has not completely solved the problem.
Tubular membranes are most efficiently manufactured using an extrusion process, so the tubular membrane cannot be tapered as readily as a disk membrane which is normally molded. The thickness of a tubular membrane is normally constant around its entire circumference. The non-uniformity of air distribution in a tubular membrane can be reduced by creating a large pressure drop across the membrane to force a more uniform distribution. However, this results in significant added energy consumption which can increase the operating costs to unacceptable levels. Therefore, the choices have been either to operate the diffuser with poor distribution or create a large head loss, neither of which is desirable from a performance or energy efficiency standpoint.
Uniform air distribution is desirable because it results in an even discharge of gas through all of the perforations. This in turn results in small gas bubbles which enhance the efficiency of gas transfer to the liquid. By using all of the perforations and uniform gas discharge through them, the gas transfer efficiency is maximized. Therefore, the number of diffusers required for a given process is minimized to reduce the equipment requirements while maintaining the required gas transfer to the liquid.